Many electronic devices, including cell phones and tablet computers, include a touch screen as an input device. A touch screen can include an array of sensors disposed over a display for detecting the proximity of objects (i.e., touches on the surface of the touch screen).
In some devices, it is desirable to track the touches of different sensed objects (such as fingers). FIGS. 11A-0/1 show a “track” 1101 of a single finger 1103 as it moves across a touch surface 1105. Finger 1103 can initially contact at position 1107. As the finger moves, various other contact positions 1109 are detected to determine the track 1101.
The single object track 1101 of FIGS. 11A-0/1 is in contrast to separate touches, as shown in FIGS. 11B-0/1. FIG. 11B-0 shows a first finger 1103 contacting a touch surface 1105 at contact position 1107. FIG. 11B-1 shows a second finger 1103′ contacting a touch surface at contact position 1111 (first finger 1103 is not contacting the touch surface 1105). Contact positions 1107 and 1111 can be associated with different “tracks”.
To better understand aspects of the embodiments, conventional capacitive sensing operations will now be described. FIGS. 12A and 12B shows a capacitance sensing system 1200. FIG. 12A is a diagram showing a sense matrix 1205 of a system 1200. A sense matrix 1205 can be divided into four sense channels (1213-0 to -3) that can be scanned simultaneously for touch events.
FIG. 12B shows system 1200 with two channels 1213-0/1 shown in more detail. A sense matrix 1205 is a mutual capacitance sensing network having capacitance sense elements (one shown as 1215) formed at the intersection of transmit electrodes (TXs) and receive electrodes (RXs). Arrows shown by dashed lines indicate a direction of scanning in each channel 1213-0/1. Scanning can start at a start element (shown as 1217) in each channel 1213-0/1 by a touch controller 1217 driving the corresponding TX lines. In the embodiment shown, TX lines can be activated sequentially moving upward in a vertical direction. A next RX line can (moving left-to-right) can be selected, and the TX activation cycle repeated.
In such a scanning arrangement, an effective scan period can vary according to an object (e.g., finger) direction and/or position. Such differences are shown in FIGS. 12C-0 to 12F-1.
FIGS. 12C-0 to 12F-1 show a portion of a sense matrix like that of FIG. 12B. All of a first channel (CH0) is shown, and a portion of a second channel (CH1) is shown. Each channel includes eight RX electrodes (RX0 to RX7, R8 to R15). Three of many TX electrodes are shown as Txi to Tx(i+2). It is assumed that a touch controller 1217 scans RX electrodes in a channel every 2 ms (approximately), moving left to right. Further, an object (e.g., finger) can be moving at about 0.3 m/s over sense elements having a size of about 5 mm.
FIGS. 12C-0/1 show an object that moves in the direction of RX scanning. In such a movement, a scanning period can be conceptualize as being 16 ms+2 ms=18 ms, as the scanning must complete a full set of RX electrodes before “catching up” to the new object position 1209.
FIGS. 12D-0/1 show an object that moves in the opposite direction to that of FIGS. 12C-0/1. In such a movement, a scanning period can be conceptualize as being 16 ms−2 ms=14 ms, as the scanning would detect the new object position 1209′ before completing a scan of all RX electrodes of the channel.
FIGS. 12E-0/1 show an object that moves in the direction of RX scanning, but across a channel boundary (from channel CH0 to channel CH1). In such a movement, a scanning period can be conceptualize as being 16 ms−14 ms=2 ms, as the new object position 1209″ occurs in a first electrode (RX8) of the next channel (CH1).
FIGS. 12F-0/1 show an object that moves in a direction opposite to RX scanning, but across a channel boundary (from channel CH1 to channel CH0). In such a movement, a scanning period can be conceptualize as being 16 ms+14 ms=30 ms, as the new object position 1209″ can be missed on a first RX scan of channel CH0 (16 ms), and then is subsequently detected on a next RX scan (14 ms).
Thus, in the above conventional scanning approach, a scanning period of a moving finger can vary from 2 ms to 30 ms, depending upon direction and/or position. Such differences can result in an object movement being interpreted as having an irregular velocity. This is shown by FIG. 12G.
FIG. 12G shows an actual track 1201 of an object (e.g., finger) on a sense matrix 1205 having an array of sense elements (one shown as 1215). It is assumed that due to scanning directions and channel divisions, scanning periods can vary as described above. Further, it is assumed that an actual track 1201 can have a relatively constant velocity, at least in the horizontal direction. FIG. 12G shows touch position determinations (one shown as 1209), made by a centroid calculation. As shown, touch positions (e.g., 1209) can reflect an irregular velocity.
The above variations on scanning direction and object location can be compounded when an object is fast moving (e.g., ˜1 to 3 m/s). FIGS. 12H-0 to 12H-7 show how a fast moving object on one track may be misinterpreted.
FIG. 12H-0 to 12H-3 show portions of a sense matrix 1205 having an array of sense elements (one shown as 1215). Each figure shows an actual object track 1201 along with detected touch position determinations (as bold boxes). Count values, which reflect a capacitance change, are also shown for each electrode.
FIG. 12H-0 shows how a fast vertical track can generate three separate contact positions. FIG. 12H-1 shows how a fast horizontal track can generate multiple positions. FIG. 12H-2 shows how a fast diagonal track can generate multiple positions. FIG. 12H-3 shows how a fast curved track can generate a “large object” (which can be discarded as an invalid touch in some processes).
Due the effects noted above, the track of a single object may be misinterpreted as two different objects at fast (high velocity) speeds, leading to “splitting” or “touch separation” (i.e., a single track misinterpreted as different tracks). FIGS. 12I-0/1 shows examples of touch splitting. FIGS. 12I-0/1 both show detected tracks from a single object track. Different tracks are delineated by solid and dashed lines. The single tracks are shown to separate in separations areas (1219).
Misinterpretation of fast moving objects can lead to erroneous inputs in touch/capacitive sensing systems.